Wireless Sensor Networks (WSN) are key parts of the Internet of Things (IOT), and they are more and more taking advantage of longer range and less interference in the sub-GHz frequency bands compared to 2.45 GHz. A challenge that then shows up is the size of the antenna, as it has to grow with lower frequencies due to the longer wavelength.
We want to keep the antenna as small as possible, but still pay attention to the “link budget”. The link budget is the total signal gain and loss between the transmitter and the receiver. It is determined by the output power of the transmitter, the sensitivity of the receiver, as well as the antenna gain at both sides, and the “Path loss”.
“Path loss” is a measure of how much signal power is lost from the transmitter to the receiver. The “Path loss” is the loss of signal in an ideal link between two isotropic antennas due to the spreading of power from the source into space. It also depends on the radio frequency due to the area of the antenna which scales with the wavelength. Therefore, the Path loss increase with higher frequencies.
Obstacles and reflections will also impact the signal loss. Lower frequencies tend to creep around corners and are therefore less affected by buildings or hills. Also note that 2.45 GHz is attenuated in humidity, such as raindrops on leaves or humidity in concrete walls. That is the reason the microwave oven use 2.45 GHz to heat up the water molecules in the food, and the frequency was deemed unusable for satellite communication.
The RF module characteristics that are important for the link budget are sensitivity and TX power. A larger negative number for sensitity is better, and a larger positive number for the TX is better.
The last item that impacts the link budget” is the antenna, which also has a large impact on the industrial design of the final product. Often a compromise must be done between an appealing design and the antenna performance. The lower the frequency, the larger the antenna should be to achieve the same gain. So, for a given link budget, the better the RF performance of the RF module, the smaller the antenna can be. A 1-2dB extra sensitivity and TX power, can reduce the antenna size by 50% which is several centimeters in the sub-GHz domain. Those centimeters will, in many cases, play a key role in allowing for a customer appealing industrial design.
Radiocrafts has released a White Paper. It will guide you to the best antenna for you system. You will also find several high performing RF modules on the site.